This book constitutes the proceedings of the 21st International Conference on Hybrid Artificial Intelligence Systems, HAIS 2026, held in Marbella, Spain, during June 18–19, 2026.

The 38 full papers included in this book were carefully reviewed and selected from 89 submissions. They are organized into the the following topical sections: Health & Biomedical AI; Applied AI: Food, Water & Resources; Vision, Robotics & Inspection; LLMs, RAG & Agents; Hybrid Optimization & Planning; Hybrid Industrial & Energy AI; and Interpretable & Robust ML.